PROJECT SUMMARY/ABSTRACT Each year, over one million adolescents are arrested; many more engage in other serious conduct problems, including binge drinking, illicit drug use, and school drop-out. Collectively, these problems result in enormous costs to individuals, families, and society. The most effective interventions to address these problems are family-based, with a strong clinical emphasis on helping parents better manage youth behavior through increased monitoring of activities, consistent use of contingencies to reward appropriate behavior, and enhancing intrinsic motivation toward prosocial behavior. Although empirically-supported treatments (ESTs), like Multisystemic Therapy (MST), exist, they are expensive and time-intensive, thus limiting their public health impact. We seek to develop, evaluate, and commercialize a linked parent-youth mobile app system (VillageWhere; VW) to support the key treatment targets of EBTs for this population: clear parental expectations, parental monitoring, discipline consistency, and parental support, while simultaneously cultivating intrinsic motivation in youth toward prosocial behaviors. When used in conjunction with an EST for delinquent youth, VW could help reduce treatment length and cost. When provided in non-EST clinical settings, VW may increase access to state-of-the-art clinical techniques to those who might not otherwise receive them. In Phase I, we developed and tested the usability, acceptability, and feasibility of VW with serious juvenile offenders and their parents receiving MST. Proof of concept was determined in two phases: a formativeevaluation, to test usability and acceptability with feedback from target end-users and stakeholders; and a summative evaluation, to test feasibility in a four-week within-subjects pilot study where parent-youth dyads used VW. Consistent with initial hypotheses, VW was well-liked by parents and teens, rated as useful, improved parental management practices, and reduced parental stress. In Phase II, we propose to accomplish the following aims: (1) complete the development of VW by expanding its features, including creating an iPhone version of VW to expand its reach and impact; (2) createan advisory board to guide additional features and processes to ensure its usability, acceptability, andreadiness for commercial success in Phase III; (3) conduct usability and acceptability tests of all new features for all target-end users; and (4) conduct a 16-week randomized controlled trial comparing VW to an attention-control mobile app to evaluate its effectiveness across four time points with 100 parent-youth dyads across the spectrum of clinically significant conduct-disorder severity. In comparison to study controls, we expect VW will significantly increase parental structuring of youth behavior, parental monitoring, greater effectiveness and consistency in providing rewards for meeting expectations, parenting efficacy, and warm parent-youth communication; and will decrease parent report of life stress as well as youths? externalizing behaviors, including reports of delinquent and substance-using behaviors.